I have a couple of questions:
How do you treat a capacitor which is common to both drains (i.e. Cgd1 is connected to Cgd2 via another capacitor) for half a circuit?

My second question, again for half a circuit (small signal), if you have an ideal current source in parallel with a resistor in the tail, wouldn't the ideal current source cause a virtual ground for the half circuit (and small signal)? What if it was a capacitor and an ideal current source in parallel?